The present invention relates to a cathode ray tube and particularly to a cathode ray tube provided with a heater having improved immunity against mechanical shock caused in the operation of welding the heater to heater supports in fabrication of an indirectly heated cathode structure and having improved immunity against adverse effects caused by thermal expansion in a manufacturing process of a cathode ray tube.
In general, color cathode ray tubes such as a color picture tube and a color display tube comprise an evacuated envelope (a glass bulb) formed of a panel portion having a faceplate, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a phosphor screen formed on an inner surface of the faceplate including a multiplicity of phosphor elements of three colors, a shadow mask having a multiplicity of apertures therein and spaced from the phosphor screen in the panel portion, a three-beam in-line electron gun housed in the neck portion for generating three electron beams and projecting the electron beams through the shadow mask to the phosphor screen, an inner magnetic shield of generally truncated pyramidal shape extending from the interior of the funnel portion into the panel portion and having openings on the shadow mask side thereof and the electron gun side thereof, and a deflection device mounted in a vicinity of a transition region between the funnel portion and the neck portion.
Three electron beams emitted from the electron gun are deflected appropriately by the deflection device, travel through the inner magnetic shield, pass through beam apertures in the shadow mask, impinge upon the phosphor screen and excite the phosphor elements of desired colors to generate light and to display a desired image on the faceplate.
The three-beam in-line electron gun housed in the neck portion includes three indirectly heated cathodes arranged in a line, and the first, second, third, fourth, fifth and sixth grid electrodes arranged in axially spaced relationship in this order on the electron beam exit side of the three indirectly heated cathodes. Each indirectly heated cathode includes a metal sleeve, a cap-shaped base metal having an electron emissive material coating on an outer top surface thereof and fitted over one end of the metal sleeve, a heater positioned within the metal sleeve, and heater supports each having a cross section of the shape of a square bracket and welded to a leg portion of the heater.
FIGS. 3A, 3B and 3C are schematic illustrations of a prior art indirectly heated cathode structure for a cathode ray tube, FIG. 3A being its cross sectional view, FIG. 3B being a plan view, partly broken away, of a heater of the cathode structure, and FIG. 3C being an enlarged view of a portion of the heater indicated by circle 60 in FIG. 3B.
In FIGS. 3A, 3B and 3C, reference numeral 31 denotes a metal sleeve, 32 is a cap-shaped base metal, 33 is an electron emissive material coating, 34 is a heater, 35 is heater supports, 36 is a heating wire, 37 is an insulating coating, 38 is a dark color coating, and 39 is leg portions of the heater 34.
The cap-shaped base metal 32 is fitted over one end of the metal sleeve 31 and is coated on its outer top surface with an electron emissive material layer 33.
The heater 34 comprises the spirally wound heating wire 36 made of tungsten (W), the insulating coating 37 made of alumina (Al2O3) and covering the heating wire 36 and the dark color coating 38 made of fine tungsten powders and covering the insulating coating 37.
The heater 34 is provided with a major heating portion formed of the heating wire 36 spirally wound and is inserted into the metal sleeve 31. The leg portions 39 of the heater 34 comprise a covered portion 39A covered with the insulating coating 37 and the dark color coating 38 and an exposed portion 39B with the heating wire 36 being uncovered. The exposed portions 39B are welded to one end of the two heater supports 35, respectively.
The metal sleeve 31 is supported concentrically with and within an outer support sleeve 40 which in turn is supported by glass beads 41.
The heater supports 35 are supported by the glass beads 41 via support studs 42 such that the major heating portion of the heater 34 is positioned within the metal sleeve 31.
The major heating portion of the heater 34 is formed of the heating wire 36 spirally wound, and each of the leg portions 39 of the heater 34 is of the three-layer winding form in which the heating wire 36 is spirally wound in three layers by doubling back the heating wire 36 upon itself at each end of the leg portion 39.
Fabrication of the three-layer winding structure of the heating wire 36 in the leg portion 39 comprises winding first the heating wire 36 spirally at a fine pitch from one end of the leg 39 to the other end thereof, then doubling back the heating wire 36 and winding it spirally at a coarse pitch from the other end thereof to the one end thereof, and again doubling back the heating wire 36 and winding spirally it at a fine pitch from the one end thereof to the other end thereof. This structure of multilayer winding of the heating wire is hereinafter referred to as the primary winding structure.
The heating wire 36 formed into the primary winding structure is again wound spirally to form the major heating portion of the heater 34 to be positioned within the metal sleeve 31. This structure of the large-diameter spiral winding of the heating wire of the major heating portion is hereinafter referred to as the secondary winding structure.
The heating wire 36 having the secondary winding structure is coated with alumina (Al2O3) except for the exposed portion 39B of the leg portions 39 of the heater 34, is covered with fine tungsten (W) powders on the alumina coating, and then is fired at a high temperature, 1650xc2x0 C., for example. The fired heating wire 36 is immersed in a mixed solution of hydrochloric acid (HCl) and nitric acid (HNO3) to dissolve molybdenum (Mo) having served as a mandrel for winding the heating wire and to complete the heater 34. The heater as described above is disclosed in Japanese Utility model Publication No. Sho 57-34671, for example.
The heater 34 has sufficient resistance to sparks and mechanical shock because of its three-layer winding structure of the heating wire 36 in its leg portions 39 and has good workability in the operation of welding the exposed portions 39B of the leg portion 39 to the heater supports 35.
Although the prior art heater of the three-layer winding structure for the indirectly heated cathode structure has a sufficiently high resistance to sparks and mechanical shock, the increased strength of the heater leg portions easily causes damages such as cracks in the insulating coating made of alumina (Al2O3) during the operation of welding the exposed portions to the heater supports.
There is a problem in that the damages such as cracks caused in the insulating coating extend and a portion of the insulating coating comes off in flakes when the heater is turned on during the operation of manufacturing a cathode ray-tube.
The flakes from the insulating coating scatter within the evacuated envelope of the cathode ray tube and degrade the performance of the cathode ray tube. The flakes stuck between electrodes of the electron gun deteriorate withstand voltage characteristics of the cathode ray tube, and the flakes stuck in beam apertures in the shadow mask of the cathode ray tube prevent phosphor elements associated with the beam apertures from luminescing.
An object of the present invention is to solve the above-mentioned problems of the prior art and is to provide a cathode ray tube which is free from peeling of the insulating coating of its heater and degrading its performance when the heater is turned on, and which is low-cost and superior in mass productivity.
To accomplish the above objects, according to a preferred embodiment of the present invention, there is provided a cathode ray tube comprising an evacuated envelope including a panel portion, a neck portion, a funnel portion for connecting the panel portion and the neck portion and a stem having a plurality of pins therethrough and being sealed to close the neck portion at one end thereof, a phosphor screen formed on an inner surface of the panel portion, an electron gun housed in the neck portion, the electron gun comprising an indirectly heated cathode structure and a plurality of grid electrodes disposed downstream of the cathode structure, spaced specified distances apart, arranged axially in a specified order, and fixed by insulating rods, the indirectly heated cathode structure comprising a metal sleeve, a base metal having an electron emissive material coating on an outer top surface thereof and attached at one end of the metal sleeve, and a heater positioned within the metal sleeve, wherein the heater comprises a major heating portion having a spirally wound heating wire and a pair of leg portions connected to opposite ends of the major heating portion, the major heating portion and an inner portion of each of the pair of leg portions on a major-heating-portion side thereof are covered with an insulating coating, the heater is welded to electrical conductors for applying a voltage thereto at an outer portion of each of the pair of leg portions, the outer portion not not covered with the insulating coating, the outer portion of each of the pair of leg portions includes a first multilayer winding portion having heating wires wound spirally in a plurality of layers, the inner portion of each of the pair of leg portions includes a second multilayer winding portion having heating wires wound spirally in a plurality of layers, a number of turns per unit length in the first multilayer winding portion is smaller than that in the second multilayer winding portion.
In the above construction, the heating wire in the exposed (uncovered) portions of the leg portions of the heater is wound with the number of turns per unit length smaller than that in the covered portion of the leg portions such that the mechanical strength of the exposed portion is made weaker. Consequently, this structure greatly reduces occurrences of damage such as cracking in the insulating coating in the covered portion of the leg portion and in a portion other than the leg portion during the operation of welding the exposed portions to the heater supports, and also greatly reduces deterioration of performance of cathode ray tubes caused by flakes coming off from the damaged insulating coating.